Magnetic Declination. 



333 



were only 11 days between apogee and perigee, or between perigee and apogee; in these cases, the 6th day 

 was counted as the 6th and 7th before and after the two epochs ; where there were 12 days of interval, the 6th 

 before apogee was counted as the 7th after perigee, and vice versa ; when there were 13 days of interval, the 7th was 

 counted as the 7th before the one epoch and after the other ; and when there were 15 or 16 days, the mean 

 westerly declination for the 7th and 8th day was used as a mean for the 7th day. The mean westerly declina- 

 tion for each of the 1 3 days so numbered was then obtained and corrected for secular change, as in the first case ; 

 the differences from the lowest mean are given below. 



TABLE II. — Mean Vai-iations of Westerly Declination after Eliminating the Secular Change, with 

 reference to the Moon's Age, Declination, and Distance from the Earth, for 1844. 



The corrections for secular change have been made at the rate of 0'-0166 -per diem, upon the supposition 

 that it is regular from day to day, which is most probable when the means of several days are taken ; the ope- 

 i-ation is similar to a transference of the projected ordinates from an oblique to a horizontal axis. 



Note. — In the discussions with reference to the moon's age, declination, and distance from the earth, it 

 should be remarked, that since 12 lunations occur in nearly the same time as 13 revolutions with respect to 

 node or with respect to apogee, any variations in the element discussed, due to changes in the moon's declina- 

 tion or distance, will be eliminated in the mean of 12 lunations ; and similarly in the means of 13 revolutions 

 with respect to declination or with respect to distance, variations related to changes of phase alone will be elimi- 

 nated ; but this is not the case in the combinations for declination and distance, with respect to each other. 



Variations of Westerly Declination with reference to the Moon's Age. — The general appearance of these 

 variations, is that of a principal maximum about 3 days after the full moon, and a principal minimum between 

 the 7th and 13th day ; there are several secondary maxima and minima, as might be expected where the varia- 

 tions are so small, and the uneliminated sources of error so considerable. It is only from a mean of several 

 years that a satisfactory result may be obtained. 



14 days to 16 days. Full Moon, 



17 20 ... 



21 24 ... 



25 28 ... 



Means of Groups. 



0'-42 

 0'-70 



c-ei 



29 days to 1 day. New Moon, 0'-46 



2 5 ... 0'-49 



6 9 ... c-ao 



10 13 ... 0'-23 



There is the appearance of a secondary minimum about new Moon, and of a secondary maximum imme- 

 diately thereafter. 



MAG. AND MET. CBS. 1844. 



4f 



